A power supply system for powering a load usually includes a controller to control an amount of power from the power supply system to the load. In some conventional power supply systems, the controller is powered directly by a relatively high voltage, e.g., ranged from 120V to 400V. Thus, the controller may need to be manufactured with a relatively high withstand voltage, e.g., up to 500V, which increases the cost and the power consumption.
In some other conventional power supply system, e.g., a power supply system 100 illustrated in FIG. 1, a transformer 110 is used to convert a relatively high input voltage VIN, e.g., ranged from 120V to 400V, to a relatively low output voltage, e.g., 15V or 20V. As shown in FIG. 1, the transformer 110 includes a primary winding 102, a secondary winding 104, and an auxiliary winding 106. A controller 130 is powered by the auxiliary winding 106. By controlling a switch 140 coupled to the primary winding 102, the controller 130 can receive a desired amount of power from the auxiliary winding 106. The range of the operating voltage for the controller 130 can be relatively low, e.g., ranged from 9V to 40V. Thus, the controller 130 does not need to have a relatively high withstand voltage.
At the beginning of the operation when the power supply system 100 is enabled, the switch 140 is off, and therefore the transformer 110 does not provide power to the controller 130. Instead, a capacitor 114 coupled to the controller 130 can provide power to the controller 130. Specifically, when the power supply system 100 is enabled, the capacitor 114 is charged by the input voltage VIN via a start-up resistor 112. A voltage VCC on the capacitor 114 starts to increase. When the voltage VCC increases to a voltage threshold VCCon, e.g., 15V, the controller 130 starts to operate, e.g., to turn the switch 140 on and off alternately. As such, a voltage V106 across the auxiliary winding 106 starts to increase. When the voltage V106 across the auxiliary winding 106 increases to a level such that the diode 118 is forward biased, e.g., when the voltage V106 is greater than the voltage VCC plus the forward-bias-conducting voltage V118 of the diode 118, the transformer 110 can power the controller 130. However, a current IOP flowing from the capacitor 114 to the controller 130 may be greater than a current I112 flowing from the start-up resistor 112 to the capacitor 114, and therefore the voltage VCC on the capacitor 114 decreases. Consequently, if the voltage VCC decreases to another voltage threshold VCC(min), e.g., 8V, before the voltage V106 across the auxiliary winding 106 increases to be greater than the voltage VCC plus the forward-bias-conducting voltage V118, the controller 130 is disabled.
In order to prevent the controller 130 from being disabled, the amount of charges stored in the capacitor 114 needs to be relatively large. In other words, the capacitor 114 has a relatively high capacitance, e.g., 100 μF. Thus, the time for the controller 130 to start up, e.g., the time for the voltage VCC to increase from 0V to the voltage threshold VCCon, e.g., 15V, is relatively long. In addition, the size of the transformer 110 is relatively large, thereby increasing the size of the printed circuit board (PCB) for the power supply system 100.